A number of digital thermometers exist which have accuracy limitations on the order of one to one hundred milli-degrees centigrade. The accuracy limitations of such digital thermometers is mainly due to quantizing error and noise of analog-to-digital convertors which follow thermistor bridges, or by inherent limitations associated with other arrangements and techniques.
The follow U.S. patents are considered by applicants to represent the closest state of the art.
U.S. Pat. No. 3,781,869 to Sudnick et al discloses a transducer amplifier with automatic balance for strain gages and other transducers in quarter, half or full bridge configurations which provides an analog output suited for measurement, control or display purposes. The amplifier includes temperature compensation provided by a silicon PN junction whose voltage-temperature characteristic is used to effect the offset of an integrated circuit amplifier stage. A signal for zero balance is derived from the amplifier output which is sampled and retained in an analog to digital converter and reconverted to analog form.
U.S. Pat. No. 3,791,214 to Keith discloses a digital clinical thermometer which includes a thermistor scaling amplifier 9 having a thermistor which is a temperature sensitive transducer.
U.S. Pat. No. 3,872,728 to Joyce et al discloses a clinical thermometer. The instrument is calibrated by placing a known resistance in a bridge circuit in place of the resistance of the line cord and the resistance within tip 36 of probe 30.
U.S. Pat. No. 4,022,063 to West et al discloses an electromechanical digital thermometer. In operation, temperature probe 10 is placed in a patient's mouth causing the thermistor therein to change resistance. This causes unbalance in bridge 13 amplified by amplifiers 14 and 15 and thus driving motor 21. Shaft 25 then takes up a new position until resistance of potentiometer 26 fed back to the bridge balances the new value of the thermistor resistance. The angular portion of the shaft 25 and coding disc 22 is indicative of the temperature of the thermistor.
U.S. Pat. No. 4,126,042 to Lynch discloses an error compensating network for a digital display thermometer in which a thermocouple is connected to one input of a differential amplifier disposed in a feedback configuration with a voltage-to-current converter for providing a signal current that is a measure of the temperature within a region being monitored by the thermocouple junction. A terminal of the thermocouple outside of the monitored region is directly connected through voltage dividing resistors to the differential amplifier and an active element of the converter to nullify the error introduced by ambient temperature variations.
U.S. Pat. No. 4,443,117 to Muramoto et al discloses a measuring apparatus wherein an amplifier is connected between a bridge circuit and an analog to digital converter followed by a data processor and display device.
U.S. Pat. No. 4,618,848 to Parfitt discloses an analog to digital converter circuit which uses a CMOS multiplexer, an analog to digital convertor and an operational amplifier buffer which is controlled by a single chip microprocessor.
U.S. Pat. No. 4,642,785 to Packard et al discloses a cordless electronic thermometer with a bridge circuit consisting of a thermistor 38 and resistors 35, 36 and 37, which feeds a signal to an analog to digital converter. A memory unit is provided for storing temperature data. A processing unit is provided for calculating the temperature of a patient.
U.S. Pat. No. 4,679,162 to Ferber et al discloses a wide range linear to log converter with a microcomputer control. The microcomputer provides slope corrections and applies the output corrections for offsets due to the circuit components and ambient temperature compensation for increased precision.
U.S. Pat. No. 4,814,692 to Baumann discloses a circuit for measuring and digitizing the value of a resistance. The resistance to be measured is both a component of an A/D converter which operates in accord with the charge balancing network and is also a component of the resistance network bridge. The circuit permits both a pure resistance measurement and attainment of a composite measured value from a plurality of measured values.
In spite of the developments and advance in the prior art, there remains a need for a high resolution digital thermometer capable of measuring temperature difference on the order of several micro-degrees centigrade.